Vehicular transmissions generally include selectively engageable gear elements for providing multiple forward drive or speed ratios through which the output torque of the engine is applied to the drive wheels of the vehicle. In automatic transmissions, the gear elements which provide the various drive ratios are selectively activated, as through fluid-operated friction torque transfer devices, such as clutches and brakes. Thus, shifting from one drive ratio to another generally involves releasing (disengaging) the torque transfer device(s) associated with the current drive ratio and applying (engaging) the torque transfer device(s) associated with the desired drive ratio. Any torque transfer device to be released during a particular shift sequence is conventionally referred to as the "off-going" torque transfer device, while the torque transfer device to be applied during that same shift sequence is referred to as the "on-coming" torque transfer device. High quality shifts are achieved only when the releasing and applying operations are properly timed and executed.
Conventionally, the shifting control effected by an automatic transmission is performed in conjunction not only with a logic control map but also various inputs which reflect such system parameters as the drive ratio at which the vehicle is presently operating, engine throttle position and engine torque. Signals representative of the various system parameters are processed in an onboard microprocessor to determine when a shift is in order and to actuate, in accordance with the logic control map, electronically operated valves in the hydraulic control system which respond to the signals received from the microprocessor in order to effect the required engagement and/or disengagement (and in the proper order) of the appropriate torque transfer devices necessary to secure the desired drive ratio changes to the output shaft of the transmission.
Conventionally, a pulse width pressure modulating solenoid valve may be employed in conjunction with each torque transfer device required to select the various drive ratios provided by an automatic transmission in order to eliminate the various problems associated with the overlap between the engagement and disengagement of the appropriate torque transfer devices. An exemplary installation of such an arrangement is disclosed in U.S. Pat. No. 4,790,513 issued to Davis et al. on Dec. 13, 1988, and assigned to the assignee of the present invention.
Installations in which a single pulse width modulating solenoid valve is employed in conjunction with a plurality of torque transfer devices are also known. See, for example, U.S. Pat. No. 4,603,603 which issued to Salmon on Aug. 5, 1985, as well as U.S. patent application Ser. No. 07/382,456, filed on Jul. 20, 1989, both of which are assigned to the assignee of the present invention. Pulse width pressure modulating solenoid valves are expensive to make and therefore add considerable cost to the hydraulic control system when an individual modulating valve is employed to control the engagement and disengagement of each torque transfer device in an automatic transmission.